fixed AttributeError in dataprep (getsmallrowdata_db)

The rdata function returning a number instead of an object leads to
additional code in other parts of the software. Should consider changing
it to return an "empty" rowingdata object.

rowers/dataprep.py

@@ -1557,7 +1557,11 @@ def getsmallrowdata_db(columns, ids=[], doclean=True, workstrokesonly=True):
     if extracols and len(ids)==1:
         w = Workout.objects.get(id=ids[0])
         row = rdata(w.csvfilename)
-        f = row.df['TimeStamp (sec)'].diff().mean()
+        try:
+            f = row.df['TimeStamp (sec)'].diff().mean()
+        except AttributeError:
+            f = 0
+            
         if f != 0 and not np.isnan(f):
             windowsize = 2 * (int(10. / (f))) + 1
         else: